The invention relates to radiation detectors and methods for detecting radiation. In particular the invention relates to the detection of neutrons in the presence of gamma-rays.
Neutrons are commonly detected using high pressure proportional counters based on He-3 and relying on the 3He2+1n0→3H1+1p1+0.764 MeV reaction. Helum-3 is used because it provides good detection efficiency for thermal neutrons, having a relatively high neutron absorption cross-section (5330 barns). These slow-moving, heavily-ionizing fragments generate a high level of ionization along their tracks in the gas compared with electrons that might be produced as a consequence of gamma-ray interactions in the detector. As a consequence, He-3-based detectors can provide relatively good levels of discrimination against gamma-radiation, except at high count-rates when pulse pile-up reduces the amplitude differences between the ionization produced by the relatively heavy neutron interaction fragments compared with that produced by gamma-ray induced photo-electrons.
Neutron detectors based on the use of cylindrical, high pressure He-3 are manufactured in a wide range of sizes. For example LND, Inc. of New York USA manufacture detectors having diameters that range from approximately 10 mm to 50 mm and lengths from 60 to 2000 mm. These can provide a sensitivity of up to 1700 cps per nv.
However, a problem with He-3-based detectors is that He-3 is in relatively short supply, and is becoming ever more expensive. There is therefore a desire for neutron detectors based on different technologies to allow for the wider use of such detectors. For example, one area where neutron detection is a valuable tool is policing the trafficking of special nuclear materials, e.g. at border crossings. Neutron detectors can be used, for example, to scan cargoes to look for neutron emission associated with the illicit transport of highly enriched uranium, or plutonium, for example.
Some known alternative approaches to neutron detection rely on reactions in Boron, Lithium and Gadolinium.
Boron
The most commonly used reaction for the conversion of slow neutrons into detectable charged particles using oron involves the 10B5 nucleus (10B5+1n0→7Li3+4α2+2.78 MeV). This reaction is frequently employed in high-pressure BF3 proportional counters. Alternatively, a gas better suited for use in a proportional counter can be used if the 10B5 is thinly deposited on the inner wall of the proportional counter so that the alpha-particles (4α2) can then escape and ionize the gas.
Boron-loaded scintillators have been made by combining B2O2 with ZnS. Boron-loaded plastic scintillators are also available. In these the plastic material has a boron content of around 5%. However, the light yield is roughly 75% that of normal plastic scintillators.
Lithium
Slow neutrons interact in the 6Li3 nucleus to produce a triton and an alpha particle. For this reaction the Q-value is 4.78 MeV (i.e. 6Li3+1n0→3H1+4α2+4.78 MeV). Since there are no gaseous lithium compounds readily suitable for use in proportional counters, practical Lithium-based neutron detectors have largely been based on scintillation counter designs. Some examples of such detectors include:
(i) Lithium Iodide Scintillation Crystals
A Europium-doped lithium iodide crystal has a scintillation efficiency that is roughly 30% that of sodium iodide. A detector having a thickness of a few millimeters provides an efficient detector for thermal neutrons.
(ii) Lithium Fluoride Loaded Zinc Sulphide with Silver/Copper Activation.
The reaction fragments from 6Li3+1n0 events interact with the ZnS scintillator to generate scintillation light photons which is detected by a photodetector. For applications in which that scintillation light is detected by a photo-multiplier tube, the ZnS is commonly doped with silver to help match the emission spectrum to the peak response of typical photo-multiplier tubes. For applications in which that scintillation light is detected by detectors having peak responses more towards the red (e.g. CCD photodetectors), the ZnS is commonly doped with copper to shift the emission spectrum to better match the response of the photodetector.
(iii) Lithium Glass Scintillators
The lithium content of some special glasses is sufficient to provide for efficient detection of thermal neutrons within a thickness of a few millimeters. However, the scintillation efficiency for lithium glass is not as high as for lithium iodide scintillator crystals.
Gadolinium
This element has a very high neutron absorption cross section (34,000 barns) such that only thin foils of gadolinium are needed to detect thermal neutrons. Some neutron detectors have been constructed by placing gadolinium foil in close proximity to a silicon detector.
Although there are a range of alternative-technologies for neutron detection available, none of these can be readily be implemented in, for example, relatively large scale and low cost detectors suitable for use for scanning for special nuclear materials, such as at border crossings.
Furthermore, there is often a need for neutron detectors to be able to operate against a significant gamma-ray background, e.g. because a smuggler may often try to mask an illicit neutron source with a legitimate gamma-ray source.
US 2009/0140150 [1] discloses an integrated neutron and gamma-ray radiation detector which distinguishes between neutron and gamma-ray detection events based on optical pulse shape processing.
U.S. Pat. No. 7,372,040 [2] discloses a broad spectrum neutron detector based on an interleaved stack of thermal neutron sensitive scintillator films and hydrogenous thermalising media. However, the detector of U.S. Pat. No. 7,372,040 [2] is designed to have negligible sensitivity to gamma-rays, which precludes its use in monitoring incident gamma-ray flux.
There is therefore a need for neutron detection schemes that may more readily be used in situations requiring relatively large scale and low cost detectors and furthermore which are capable of providing a constant neutron-detection sensitivity even when subjected to high gamma-ray dose rates.